the voltech handbook of pwm motor drives

7/31/2019 The Voltech Handbook of Pwm Motor Drives

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Voltech Application Note 108

Product: App-Note 108 Issue 1.0 VPN: 86-646

TheVOLTECH HANDBOOK

ofPWM MOTOR DRIVES

Andrew Tedd


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PWM Motor Drives Issue 1.0 Page 2 of 53

Contents

1. Introduction ............................................................................... 32. Principles of PWM motor drives ................................................ 73. Characteristics of PWM motor drive ........................................ 13

3.1 Voltage-frequency relationship .......................................... 133.2 Choice of Carrier frequencies ............................................ 143.3 Effect of harmonic and carrier frequency components....... 14in the PWM output ................................................................... 14

4. Measurements required on PWM motor drives ....................... 175. Motor Output Measurements ................................................... 19

5.1 Sensors ............................................................................. 195.2 Torque ............................................................................... 195.3 Speed ................................................................................ 235.4 Motor Output Power .......................................................... 265.5 USB-6009 Connections ..................................................... 275.6 Mechanical Power ............................................................. 28

6. Drive Output Measurements ................................................... 31

6.1 Drive output measurements using the PM6000 ................. 337. Drive DC Bus Measurements .................................................. 358. Drive Input Measurements ...................................................... 379. Loss and Efficiency Measurements ......................................... 4110. Examining Detail Under Dynamic Load Conditions ............... 43Appendix A: PM6000 VPAS Operating Facts .............................. 45Appendix B: Required number of watt-meters ............................. 47Appendix C. Current Connections on PWM Drives ..................... 49Appendix D. Instantaneous Power in a 3-phase system. ............ 51


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1. Introduction

Three-phase AC motors have been the work-horse of industry

since the earliest days of electrical engineering. They are reliable,efficient, cost effective, and need little or no maintenance. Inaddition, AC motors such as induction and reluctance motors needno electrical connection to the rotor so can easily be madeflameproof for use in hazardous environments such as in mines.

The main disadvantage of AC motors is that, when supplieddirectly from the three-phase AC line, they are essentially aconstant speed drive. This is because the fixed frequency supply

to the stator windings creates a magnetic field rotating at constantspeed. This rotating magnetic field turns the rotor at nearly thesame speed - the difference in speed is known as motor slip.

Figure 1

For efficient operation the motor works with a slip of just a fewpercent, i.e. the motor operates at constant speed.

Early attempts to provide an adjustable speed drive on an ACmotor were by reducing the applied AC voltage using variable

resistors or by an autotransformer in the AC line: -

Figure 2


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As the applied voltage is reduced, the motor provides less torque,so the motor slows down. Provided that the torque required by themotor is less at lower speeds, such as for a pump or fan, then thistechnique provides a variable speed drive. Even so, when the

motor is operating at reduced speed, there is a very large slipbetween stator and rotor and the motor will be operating at muchreduced efficiency. In addition, loads that require full torque atreduced speed cannot use this method of speed control.

For these reasons industrial variable speed drives have beendominated by DC motor drives. The commutator in the DC motoracts to provide a rotating field that is matched to the speed ofrotation, so that speed control at full torque can be achieved

merely by adjustment of the applied DC voltage. DC motors,however, are larger than AC motors, are more expensive, needmuch more maintenance, and are difficult to use in hazardousenvironments. They do however produce an excellent variablespeed drive.

Figure 3

In order to provide proper speed control of an AC motor, it isnecessary to supply the motor with a three-phase supply of whichboth the voltage and the frequency can be varied. Such a supply

will create a variable speed-rotating field in the stator that willallow the rotor to rotate at the required speed with low slip. ThisAC motor drive can efficiently provide full torque from zero speedto full speed, can over speed if necessary, and can, by changingphase rotation, easily provide bi-directional operation of the motor.A drive with these characteristics is known as a PWM (PulseWidth Modulated) motor drive.


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Figure 4

Although the principles of PWM drives has been understood forsome years, advances in the technology of power semi-conductors, control electronics, and microprocessors has greatly

stimulated the use of such drives. This has been furtheraccelerated by the use of vector control methods, which give theAC drive the capability and flexibility of a full DC motor drive. PWMmotor drives have become the dominant method of variable speedmotor control, and are being used not only in industry, but inapplications as diverse as electric vehicles and domestic airconditioners.

PWM drives produce complex waveforms, both on their output to

the motor, and also in the electrical supply to the drive. ThisApplication Note is designed to simplify electrical measurementson these drives.


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2. Principles of PWM motor drives

A block diagram of the essential elements of a PWM motor drive isshown in figure 5.

Figure 5

The three-phase supply is rectified and filtered to produce a dcbus, which powers the inverter section of the drive. The inverterconsists of three pairs of semiconductor switches (MOSFET,

GTO, power transistors, IGBT etc, with associated diodes. Eachpair of switches provides the power output for one phase of themotor.

Within each pair of semi-conductor switches, (for example A), theoutput is connected either to +V via switch A+ and it's anti-paralleldiode, or to -V via A- and its diode. In this way there is always a bi-directional path for current flow to the motor windings for bothpositive and negative output voltages: -

+ve voltage+ve current

+ve voltage-ve current

-ve voltage-ve current

-ve voltage+ve current

Figure 6


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Each pair of semiconductor switches is driven by the controlelectronics so as to generate a high frequency square wavecarrier pulse waveform at each of the phase outputs: -

Figure 7

As the carrier is identical on all three phases, the net voltageappearing across any phase of the motor windings due to thecarrier alone will be zero e.g.: -

Figure 8

The carrier is said to be un-modulated, and no drive power is

applied to the motor.

In order to drive the motor, the control electronics generates threelow frequency sine waves, 120 apart, which modulate the carrierpulses to each pair of switches. The width of positive and negativepulse within each carrier cycle is modulated according to theamplitude of the low frequency sine waveform of that phase. Forexample: -


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Figure 9a

Figure 9b

The voltage appearing across one phase of the motor winding istherefore: -

Figure 10


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It can be seen that the average voltage presented to the motorwinding is approximately sinusoidal. The two other phases of themotor winding will have similar average voltages spaced 120apart.

It is clear that although the pulse-width-modulated voltagewaveform applied to a motor winding contains a component at therequired frequency, it also contains a number of other, higherfrequency components. For example, the phase-to-phasewaveform in figure10 has a frequency spectrum as follows:

Figure 11

Fortunately, to a large extent, the motor appears as an inductor tothe output voltages of the inverter. As an inductor has higherimpedances to higher frequencies, most of the current drawn bythe motor is due to the lower frequency components in the PWMoutput wave-shape. This results in the current drawn by the motorbeing approximately sinusoidal in shape.

Figure 12


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By controlling the amplitude and frequency of the modulatingwaveforms, the PWM drive can output to the motor a three-phasesupply at the necessary voltage and frequency to drive the motorat any required speed.


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3. Characteristics of PWM motor drive

3.1 Voltage-frequency relationship

The characteristics of a PWM motor drive can best be understoodby considering a simplified equivalent circuit of one motor phase(figure 13):

Figure 13

In this circuit, R and L represent the resistance and inductance ofthe motor as seen by the supply, and E represents the back EMFproduced by motor rotation.

The magnitude of the back EMF, and its frequency, is proportionalto the motor speed. The PWM drive must therefore adjust both thevoltage and the frequency to adjust the speed, with an outputvoltage slightly higher than the back EMF to force current throughthe impedance of R and L. In fact the drive electronics needs toproduce a voltage/frequency characteristics as shown in figure 14.The offset in this characteristic is to overcome the voltage drop inthis impedance and allow the drive to deliver the required currenteven at low or zero speed.

Figure 14


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3.2 Choice of Carrier frequencies

Most PWM drives operate with a fixed carrier frequency that isseveral times higher than the highest output frequency that is to

be used. As industrial drives operate with an output frequencyfrom a few Hertz up to about 100Hz, they use a carrier frequencyin the range of 2kHz upwards.

As power semiconductors improve, the trend is to increase carrierfrequencies up to ultrasonic frequencies (18kHz+), and beyond,but this brings both advantages and disadvantages: -

High Carrier Frequencies

Advantages DisadvantagesLower losses in motor(current more sinusoidal)No audible noise due to carrier

Higher switching losses ininverterPotential for more radiated radiofrequency noise.

The choice of carrier frequency is therefore a compromise, andcareful measurements must be made on both the input and theoutput of the drive to make the optimum choice.

3.3 Effect of harmonic and carrier frequency componentsin the PWM output

It was shown earlier, that although the PWM output voltagecontains a large number of frequency components other than thefundamental (or wanted component) that these components aregenerally of higher frequency and suppressed by the inductanceof the motor winding.

In practice, as shown by the equivalent circuit of figure 13, themotor is not a simple inductor, but is dominated by the back EMF.

Unfortunately, as the back EMF generated by the motor isessentially a sinusoidal voltage at the fundamental frequency, itprovides no opposition to the flow of current at the harmonic andhigher frequencies. For this reason these currents are larger whencompared to the fundamental than would be the case if the motor

were a pure inductor.


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It is important therefore, that the strategy employed in themodulation of the carrier frequency is such as to produce in thewindings a current that is as sinusoidal as possible. In particular,care must be taken to minimize the level of low order harmonic

voltages produced, as the impedance of the motor to thesevoltages will be very low. In practice then the drive produces inthe motor: -

a). A 'wanted' component of current at the fundamentalfrequency.

b). 'Unwanted' components of current at frequencies that aremultiples of the fundamental frequency (these are harmonics) and

also components of current at frequencies related to the carrierfrequency.

The 'unwanted' components in the motor current have two effectson the motor: -

1. Components of current other than the fundamental representadditional currents in the motor stator and rotor windings,creating additional heat and reducing motor efficiency.

2. The 'unwanted' components create magnetic fields in thestator that may have negative or zero phase sequence,producing negative or braking torque. This can substantiallyreduce the amount of power available from the motor.

The effects of these unwanted components on the operation of themotor can be expressed by measurements of the fundamental andtotal output power of the inverter, by harmonic analysis of thevoltage and current waveforms, and by torque/speed

measurements on the motor.

The only useful power delivered to the motor is at the fundamentalfrequency - any power associated with the harmonics or carrierfrequency does not contribute to the useful work done by themotor. The most efficient PWM drives are those that not onlyminimize losses in the converter, but also generate the most purecurrent waveforms to minimize power and torque losses in themotor itself.


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4. Measurements required on PWM motor drives

Figure 15

The following are examples of useful measurements to be made

on PWM motor drives: -

Drive Section Parameters App NoteSection

Motor OutputMeasurements

Speed, Torque, Shaft Power Section 5

Drive OutputMeasurements

Total Output Power & Power FactorFundamental Output Power & P FRMS Output Voltage and CurrentFundamental Output Voltage and CurrentHarmonic voltages, currents & powers.

Output Frequency

Section 6

Drive DC BusMeasurements

DC Bus Voltage, Current and Power Section 7

Drive InputMeasurements

Input Voltage and CurrentInput Power and Power FactorInput VA and VArsInput Harmonic Currents (includingchecking to harmonic specifications suchas IEC61000-2-2)

Section 8

Efficiency Measurements Efficiency of each section of PWM drive,motor efficiency and overall efficiency

Section 9

Measurements UnderDynamic LoadConditions

Real time analog outputs representingvoltage, current, watts and power factor ofdrive output.

Section 10

The following sections 5 10 show measurements and screenshots from the PM6000 VPAS software (Voltech Part Number: 28-312). A 30 day trial of the software can be downloaded fromVoltechs web site (www.voltech.com). To install this softwaredouble click onto PM6VPAS.exe (this is a self extracting autoinstall file).


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5. Motor Output Measurements

Motor output measurements can be made by installing torque andspeed transducers on the output shaft of the motor: -

Figure 16

5.1 Sensors

Torque and Speed transducers produce an electrical signal that isproportional to the motor speed and the motor torque. Bymeasuring these the motors output power or mechanical powercan be determined.

5.2 Torque

5.2.1 What is Torque?

Torque is a force that tends to rotate or turn things. You generatea torque any time you apply a force using a spanner or wrench.Tightening the nuts on the wheels of your car is a good example.

When you use a spanner or wrench, you apply a force to thehandle. This force creates a torque on the nut, which tends to turnthe nut. English units of torque are pound-inches or pound-feet;the SI unit is the Newton-meter. Notice that the torque unitscontain a distance and a force. To calculate the torque, you justmultiply the force by the distance from the center. In the case ofthe nuts, if the spanner or wrench is a foot long, and you put 200pounds of force on it, you are generating 200 pound-feet of torque.If you use a 2-foot wrench, you only need to put 100 pounds offorce on it to generate the same torque.


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The torque of a motor is the rotary force produced on its outputshaft, it is a twisting force that rotates or turns an object, like awheel. The motor torque is measured in Newton-meters (Nm) orFoot-Lbs (1 Foot-Lb = 1.3558Nm). Torque ratings vary from less

than one Nm for small motors, to several thousand Nm for largemotors.

5.2.2 Measuring Torque

Torque can be measured by rotating strain gauges as well as bystationary proximity, magnetostrictive and magnetoelastic sensors.All are temperature sensitive. Rotary sensors must be mounted onthe shaft, which may not always be possible because of space

limitations.

Figure 17

A strain gauge can be installed directly on a shaft. Since the shaftis rotating, the torque sensor can be connected to its powersource and signal conditioning electronics via a slip ring. Thestrain gauge can also be connected via a transformer, eliminating

the need for high maintenance slip rings. The excitation voltage forthe strain gauge is inductively coupled, and the strain gaugeoutput is converted to a modulated pulse frequency (Figure 17).

The maximum speed of such an arrangement is 15,000 rpm.Strain gauges also can be mounted on stationary supportmembers or on the housing itself. These "reaction" sensorsmeasure the torque that is transferred by the shaft to therestraining elements. The resultant reading is not completely

accurate, as it disregards the inertia of the motor.


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Alternatively torque transducers can be purchased to suit themotor that they are intended for as they are sold with minimumand maximum Newton-meters (Nm). Many of these transducershave two outputs, one for torque and one for speed and are

available as non-rotary (no attachment to motor shaft) and rotarysensors (attached to motor shaft).

5.2.3 Using the PM6000 VPAS Software to Measure Torque.

Note: See section


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5.5 USB-6009 Connections, for information on making analogmeasurements with a PM6000.

Figure 18 shows the USB-6009 set-up via the PM6000 VPAS

software. The sensor used was a DC 0 - 10V output scaling with0.4V = 1Nm.

Figure 18


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Figure 19 shows the output of the torque set up.

Figure 19

5.3 Speed

5.3.1 What is Speed?

Motor speed is commonly described as revolutions perminute(abbreviated rpm, RPM) and is a unit of frequency i.e. the number

of full rotations completed in one minute around a fixed axis.

5.3.2 Measuring Speed

A speed sensors output is sometimes an analog outputproportional to speed, but more commonly the output is TTL pulseproduced by a disc on the motor shaft. The holes are aligned witha light source and photosensitive diode or transistor (i.e. on, offproducing a positive and negative going TTL pulse or square wave

as the shaft rotates). By measuring the frequency of the TTLsignal from the rotating disc, and applying a scaling factor, themotor speed, in RPM, can be determined.

For example, if the rotating disc produces n pulses per revolution,the RPM can be calculated as:

60Pulses per second x

n


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5.3.3 Using the PM6000 VPAS Software to Measure Speed.

Note: See section


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5.5 USB-6009 Connections, for information on making analogmeasurements with a PM6000.

Figure 20 shows the USB-6009 set-up via the PM6VPAS software.

The rotating disc used had 18 holes and so a scaling factor of60/18 or 3.333 was used.

Figure 20

Figure 21 shows the output of the speed set up.

Figure 21


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5.4 Motor Output Power

The speed and torque transducers produce electrical signals

proportional to the motor speed and motor torque. From thesemeasurements the motor output power can be calculated.

Motor Output Power (W) = Torque (Nm) x Speed (radians/sec)= Torque (Nm) x Speed (RPM) x /30

Note :

1 Lb-ft = 1.3558 Nm

1 HP = 745.7W


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5.5 USB-6009 Connections

The USB-6009 is a multi function I/O providing an interface withthe PM6000 alongside PC software called PM6VPAS, which

allows the correct measurements of any analog input within thelimitations of the device. These can include torque, speed,mechanical power and efficiency. The output of each sensor iseither an analog voltage or a TTL pulse. THE USB-6009 catersfor both types of signal.

If your speed sensor output is a TTL pulse then connect it to pins29 and 32. If your speed sensors output is a analog voltage (+/-20V max.) then connect your sensor to CH1s differential input,

pins 2 and 3.

Your torque sensor will be either differential or non-differentialtherefore connect this sensor to CH2 pins 4 and 5 or pins 5 and 6(+/- 20V max.).

Figure 22

If you set channel 1 on the PM6000 VPAS to be a pulse input,then you cannot use channel 1 on the USB-6009 as an input. Thesame applies for channels 2, 3 and 4.

Pulse Sensor

CH1 Non-differential inputDifferential input





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5.6 Mechanical Power

The power of a motor is the product of its torque and speed. ThePM6000 VPAS software computes the motor power using the

formula in section 5.4 to display the power of the motor output asshown in figures 23 and 24.

Figure 23 shows the USB-6009 set-up via the PM6000 VPASsoftware. The sensor used was as in Torque (section 5.2) andSpeed (section 5.3). Note the scaling factor on the mechanicalpower reading is /30.

Figure 23


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Figure 24 shows mechanical power, in Watts.

Figure 24


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6. Drive Output Measurements

The output waveform of a PWM drive is very complex, consistingof a mixture of high frequency components due to the carrier and

components at low frequency due to the fundamental.

The problem this poses for most power analyzers is that they caneither measure at high frequencies, in which case low frequencyinformation in the waveform is lost; or else they filter the PWMwaveform to measure at low frequencies, in which case highfrequency data is lost.

The difficulty occurs because the waveform is being modulated at

low frequency. High frequency measurements, such as total rmsvoltage, total power etc, must therefore be made at high frequencybut over an integral number of cycles of the low frequencycomponent in the output waveform.

Figure 25

The PM6000 overcomes this problem by using a special operatingmode for PWM output measurements. The data is sampled at high

speed, and TOTAL quantities, including all harmonic and carriercomponents, are computed in real time. At the same time, thesampled data is digitally filtered to provide low frequencymeasurements such as FUNDAMENTAL and measurement ofoutput frequency.


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Figure 26

As well as the advantage of getting both low and high frequencyresults from the same measurement, this technique allows thehigh frequency measurements to be synchronized to the lowfrequency signal, which is the only way of providing high frequency

measurement results that are both accurate and stable.

There is a choice of three filters to select according to the outputfrequency range to be measured (see figure 27): -

Filter Application

5Hz PWM Drives down to 5Hz output

1 Hz Low speed measurement downto 0.5Hz

0.1Hz Very low speed measurement down to

0.1Hz

Figure 27


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The choice of filter does not effect measurement of the higherfrequency components as these measurements are made onunfiltered data. Low frequency measurement results are availablewithin the bandwidth of the selected filter.

6.1 Drive output measurements using the PM6000

The instrument is wired to the output in the 3 phase 3 wireconfiguration. (also known as two wattmeter method - refer toAppendix B for the theoretical proof that the power delivered to asystem via 'n' wires may be measured with 'n-1' watt-meters). ForPWM drives up to 30A rms output current, the PM6000 may bewired directly into the drive output as shown below.

Figure 28

Drives with greater than 30A rms output current may be wired tothe PM6000 as suggested in Appendix C.

In addition, since the PM6000 can have as many as 6 channelsfitted, additional channels are available as independent channelsto make simultaneous measurements of other sections of the

circuit such as the DC bus within the PWM drive, as described insection 7.

It is a particular advantage of the PM6000 that it can be wireddirectly to PWM outputs for measurement on lower current drives.This is because, while AC current transformers and Hall - effectCT's provide good accuracy with higher currents, they tend to givepoor results with currents of just a few amps. Best accuracy at lowcurrent is achieved by using a direct connection to shunts, but this

causes problems for most power analyzers as the shuntmeasuring system cannot tolerate the high common mode


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voltages that exists on PWM drive outputs. The voltage across theshunt may be only a few milli-volts, but the shunt potential ismoving up and down with respect to the ground by hundreds ofvolts at several kV per microsecond. The PM6000 input circuitry

provides superb rejection of these common mode voltages, whichcause other analyzers to give totally erroneous readings.

Although only two channels of the PM6000 are used formeasurements using the two wattmeter method, the instrumentwill vectorially compute and display values for the current in thethird (unmeasured) wire. This is a valuable check on the balanceof the load.

The instrument will now measure the output power with theselected filter. Verify that the measured frequency is correct. If thePM6000 is having difficulty with measuring the frequency, ensurethat the correct filter frequency range has been specified.

Note that the Vrms, Arms, and Watts figures are measured frompre-filtered values and therefore include all high frequencycomponents whereas the fundamental values will measure onlythe values, which contribute to work in the motor. It is normal tohave a large difference between the rms and fundamental voltage,and usually there would be a smaller difference for current andwatts as the inductive motor filters the current.

The high frequency losses may be estimated by the differencebetween the total watts and the fundamental watts read on theSUM channel. This represents electrical power delivered by thePWM drive, which does not contribute to the mechanical outputpower and therefore adds to the heating of the motor:

High Frequency Losses = Total Watts - Fundamental Watts

This is a useful measurement when comparing PWM drives.


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7. Drive DC Bus Measurements

Figure 29

Although the link between the input and the output section of thePWM drive is referred to as the dc bus, the voltage and current inthis bus are far from pure dc, so care must be taken in making thenecessary measurements.

DC bus measurements are best made on the input side of thestorage capacitors, as shown in figure 29 as the current here isessentially low frequency capacitor charging pulses from the acsupply, and free from the high frequency current pulses that may

be drawn by the inverter section.

If dc bus measurements are made on their own, CH1 of thePM6000 can be used. However, dc bus measurements are oftenmade in conjunction with other three phase three wiremeasurements such as the input or output of a drive. In this casechannels 1 and 2 could be used for the drive input, channel 3could be used for the dc bus and channels 4 and 5 could be usedfor the drive output.


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Suggested measurements for DC bus.

Measurement Reason

W Total power in dc bus.

Can be used inefficiency calculations.

Arms RMS charging currentin DC bus. Useful forsizing conductor orfuses.

ADC DC component ofcurrent in dc bus. Thisis smaller than Arms.

VDC Mean voltage acrossstorage capacitor.

Vpk Peak voltage acrossstorage capacitor.

Figure 30


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8. Drive Input Measurements

The input circuitry of most PWM motor drives is essentially athree-phase diode rectifier bridge with capacitor filter: -

Figure 31

The input current to such a circuit consists, on each input phase,of pulses of current that charge the storage capacitor: -

Figure 32

The input current is therefore a distorted current waveform, with afundamental component at supply frequency, but withconsiderable harmonic content.


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If the inverter section of the drive presented a constant-currentload to the input circuitry, the input current on each phase andwould be then a distorted waveform at constant amplitude: -

Figure 33

The input current drawn is essentially independent of the PWM

output frequency since, as shown in Appendix D, theinstantaneous power drawn by the drive is a constant, andtherefore the current required from the input to charge thecapacitor on the dc bus is a constant.

The PM6000 is wired to the input in 3-phase 3-wire configurationas shown in Figure 34 (also known as two wattmeter method -refer to Appendix C for the proof that the power delivered to asystem via 'n' wires may be measured with 'n-1' watt-meters).

Figure 34

In this wiring configuration it is possible to use a third channel ofthe PM6000 as an independent channel to measure, for example,

the dc bus within the PWM drive.


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Connect the PM6000 channel 1 and 2 to the inout of the PWMdrive in the three phase three wire configuration as shown inFigure 34.

If the readings vary too much, due, for example to a single phasePWM drive being measured, set the averaging to a value of 10 orgreater.

The PM6000 will now measure the input parameter, includingpower and harmonic content. The frequency displayed will be theac supply frequency.


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9. Loss and Efficiency Measurements

On any system, measurements of losses and efficiency are alwaysbest made by making simultaneous measurements on the input

and output of the system.

Figure 35

This is particularly important for systems with high efficiency, suchas PWM drives. This is because, if separate measurements aremade on input and output, and the system is shut down betweenmeasurements to transfer instrumentation, one cannot always be

certain that exactly the same load conditions exist for bothmeasurements. Any unnoticed difference in load conditions willappear as a major error in measured losses.

For example:

Set up Number 1 Measure Input.

Figure 36

Switch off system, reconnect for output measurements, and switchon again: -


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Set up Number 2 Measure Output (but conditions have slightlychanged).

Figure 37

Apparent losses = 1052.6 - 1020w = 32.6w.

Actual losses = 1073.7 - 1020w = 53.7w.

This represents a very substantial error in measured losses!

A modern PWM drive is a high efficiency system, and it isrecommended to make all measurements simultaneously to obtainthe most accurate efficiency reading. A PM6000 5 channelinstrument will allow measurements on all parts of the drivesystem simultaneously: -

Figure 38

Using Voltechs PM6000 VPAS software the input group, the DCbus, the output group and the auxiliary measurements can all bedisplayed simultaneously allowing for accurate, real-time efficiencymeasurements, including mechanical power and efficiency.

A+

A-

B+

B-

C+

C-

V-

V+

AB

C

Input Measurements DC Bus Measurements

Drive Output

Measurements

CH1 CH2 CH3

PM6000

CH4 CH5 CH6 AUX

PM6000


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10. Examining Detail Under Dynamic Load Conditions

The PM6000 VPAS software (visit www.voltech.com for a 30 daytrial), allows you to view a number of parameters from a system onyour PC. Figure 39 shows an example of reading made on a three

phase input, with detailed graphs of the voltage, current and Wattsharmonics.

Figure 39

The data can also be logged directly to a file which will enable theuser to analysis the performance of the PWM drive over time.

Example ofCH1sharmonicsmeasured

on themains input


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Appendix A: PM6000 VPAS Operating Facts

Synchronization of results between groups and betweentorque/speed is 50mS. The PM6000 is read 50mS after the torque

and speed has been measured. The same is true between groups.

Data logging provides two methods: -

a). Interval data logging (minimum interval 5 seconds, maximuminterval 1 hour, stops after a selected quantity of results).b). Continuous data logging (logs data from 1/3 second to 5seconds, dependent on how many groups and how many resultsare being displayed). Example 300-500 results update approx. 1

update per second. 100 200 results update approx 1/3 to 1/2second updates.

Software updates are dependent on the amount of groups andresults being displayed. Example 300-500 results update approx.1 update per second. 1 200 results update approx 1/3 to 1/2second updates.

The total amount of readings that can be displayed is: -

a). 3P4W group = 978 results (updates every 2 seconds).b). 3P3W group = 654 results (updates every 1.5 seconds).c). 1P2W group = 324 results (update every 1 second).

d). Maximum number on 2 groups of 3P4W = 1956 results(updates every 4 - 5 seconds).


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Appendix B: Required number of watt-meters

Consider the situation shown below, where the circuit is poweredby 'n' wires.

Figure B-1

Each wire will have an associated voltage (V1 to Vn) with respectto a separate reference, and an associated current (I1 to In).. Thetotal instantaneous power, P, will be given by:

P = V1I1 + V2I2 + ... + VnIn

Applying Kirchhoff's first law, that the total current flowing into acircuit must equal the current flowing out, allows us to write the nthcurrent in terms of all the others:

In = - 11 12 - ... In 1

Substituting this expression for In in the equation for the powergives:

P = (V1 Vn)I1 + (V2 Vn)I2 + ... + (Vn 1 Vn)In 1

This equation shows that the total power can be measured with n-1 watt-meters (i.e. voltmeter-ammeter pairs) connected as shownbelow, where the nth wire has now taken on the function of a''common'' or ''Lo'' terminal.


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Figure B-2

It does not matter which of the wires is assumed to be thereference terminal, as the expression for power is symmetrical for

all nodes.

Clearly, the general result which falls out from the final equationfor the power measured is as follows: -

''In a circuit powered by several wires, the total associated powercan be measured using a number of watt-meters equal to: - (thenumber of wires -1).''


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Appendix C. Current Connections on PWM Drives

PWM motor inverters range in power output from a fraction of akilowatt up to several hundred kilowatts, representing a wide rangeof motor currents.

Up to 30 Arms phase current, direct connection to the PM6000current shunt input terminals (Ahi and Alo) can be made. Forcurrents above this level, it is necessary to use either:

(i) A shunt connected to the PM6000 EXTERNAL current inputconnector. The output of the shunt should be in the range of 6mvto 2.5v peak for the range of measured current.

or

(ii) A current transformer with secondary connected to either thedirect current input or the External input. The choice will dependon the secondary output of the CT; it may be voltage or current

output, and it may be more convenient to convert a current outputto voltage with a precision resistor if its level is too low for thedirect current input (less than 20mA).

Both methods can be used successfully in motor applications, themeasurements being scaled automatically by the PM6000 to givedirect readings of motor current.

The PWM motor measurement environment presents a need for awide range of frequency capability from the current transducer:Operation at motor frequencies from as low as 1 Hz need to bestudied, and switching frequencies beyond 10kHz can be presentin the current waveform.

Most current transformers are designed and rated for operation atfrequencies above 45Hz, and not all have a good high frequencyresponse. Accuracy and phase performance are degraded outsideof the defined working range of the CT.


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A range of current modules manufactured by the Swiss companyLEM overcome these problems by using a field nulling techniquein the core. This provides a wide dynamic range from DC to wellbeyond 100kHz, with accuracies better than 0.5% of nominal

current.

These current modules have active inbuilt circuitry, which requiresa power source to drive them.

The accuracy of the LEM modules are rated as percentage of thenominal current, so to measure currents lower than the nominalvalue, the best accuracy is achieved by winding several primaryturns. For example, if an LT200-S is being used and a maximum

current of 100A is to be measured, make 2 windings of the primarycable.


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Appendix D. Instantaneous Power in a 3-phasesystem.

Consider a 3-phase load with an angular frequency where the

current on each phase is lagging the voltage by a phase angle, .

Figure D-1

Where:

V1(t) = Vp sin ( t 0)

V2(t) = Vp sin ( t 2 3 )

V3(t) = Vp sin ( t4

3

)and

A1(t) = Ap sin ( t 0 )

A2(t) = Ap sin ( t 2 3 )

A3(t) = Ap sin ( t 4 3 )

Where Vp and Ap are the peak of the voltage and currentwaveforms respectively.

The instantaneous total power P(t) is given by:

P (t) = V1(t) A1(t) + V2(t)A2(t) + V3(t) A3(t).

= VPAP [ sin ( t 0). sin ( t 0 ) ]

+ VPAP [ sin ( t2

3 ). sin ( t 2 3 ) ]

+ VPAP [ sin ( t4

3 ). sin ( t 4 3 ) ]

Now, sinA sinB = 12 [ cos (A B) - cos (A + B) ]


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P (t) =V A

2

P P

[ cos cos (2 t ) ]

[+ cos cos (2 t 4 3 ) ]

[+ cos cos (2 t 2 3 ) ]

3 cos [cos (2 t ) (2 t )

+ cos (2 t

cos

)]

2

43

3

Now, cos (2 t ) + cos (2 t 4 3 ) + cos (2 t 2 3 ) = 0for any value of t. This term represents the summation of three

vectors 120 apart.

P (t) =3V A

2

P P cos .

This term is not a function of t, so the instantaneous power in athree-phase system is a constant.


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the voltech handbook of pwm motor drives

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